1. Field of the Invention
The present invention relates to a blade device which contacts an object in, for example, an image forming apparatus and also to an image forming apparatus using such a blade device.
2. Description of the Related Art
A cleaning device which contacts and cleans an image carrying surface of a component of an image forming apparatus will be described, as a typical example of the blade device to which the present invention pertains.
In general, an image forming apparatus such as a copying apparatus incorporates an image carrier. The surface of the image carrier is initially charged uniformly and is selectively exposed to light from an original image so that an electrostatic latent image is formed on this surface. This latent image is then developed to become a visible image by means of a toner, and thus formed toner image is transferred to a recording member, whereby the original image is recorded. Any residue remaining on the image carrier after the transfer of the image, such as the paper dust, lubricant contained in the toner and so forth, are removed by a cleaning device.
A typical cleaning device includes a cleaning blade made of an elastic material such as urethane rubber. The blade is held in contact with the image carrier during rotation of the image carrier, so as to scrape the above-mentioned residue off the image carrier. The toner which is being scraped serves as a lubricant between the image carrier surface and the blade edge, so that the cleaning blade can maintain a constant position relative to the image carrier during the scraping.
It is to be understood, however, that the image having an extremely low image density is formed, friction between the image carrier and the blade is drastically increased due to shortage of the toner which would function as the lubricant. Consequently, the end of the blade tends to be turned up or locally damaged as the image carrier rotates. This problem is serious particularly when the image carrier is an OPC photosensitive drum having a surface layer made of a polymeric resin such as polycarbonate, because the blade made of urethane rubber exhibits a very large coefficient of friction when used in contact with such a polymeric resin, thus enhancing the tendency of turnino up or local damaging of the blade edge. This problem also is serious in full-color copying apparatus in which the blade has to be kept in contact with the image carrier at a higher contact pressure than in ordinary copying apparatuses.
FIG. 9 shows an arrangement proposed for the purpose of the above-described problem. In this arrangement, a toner reservoir 52 is disposed upstream of a cleaning blade 50 as viewed in the direction of rotation of an image carrier 51, so that the toner 53 scraped off by the blade 50 is brought into contact with the image carrier 51 so as to function as the lubricant.
The arrangement shown in FIG. 9, however, is still unsatisfactory in that only a small amount of the toner is collected in the reservoir when a number of images of low levels of image density are successively formed. In such a case, the above-mentioned problems such as turning up or local damaging of the blade edge are caused due to insufficient lubrication. In particular, a large friction between the blade 50 and the image carrier 51 is developed in the regions outside the image forming area, typically at both longitudinal ends of the blade 50, because the toner which would function as the lubricant does not exist in such regions. Consequently, the blade 50 serves as a brake which brakes the rotating image carrier 51, thus making the rotation of the image carrier 51 unstable, while allowing vibration or turning up of the edge of the cleaning blade 50.
It has also been proposed to reduce the friction by applying a fluoro-resin powder such as PTFE or PVD on the end of the blade 50 where the blade 50 contacts the image carrier 51. Such fluoro-resin powder, however, is consumed away during long use of the copying apparatus, resulting in an increase in the friction between the blade 50 and the image carrier 51.
Rubbers also are often used as the material of the elastic blade. A blade made of a rubber, however, surfers from a problem in that the blade becomes fragile due to hydrolysis so that the surface of the blade undesirably allows the residue on the image carrier to pass the nip between the blade and the image carrier. Even an urethane rubber, which exhibits a comparatively small tendency of degradation due to hydrolysis, experiences inferior cleaning due to hydrolysis as shown below, after 2 to 3 years of continuous use. EQU (O--C--NM (P)+H.sub.2 O) OH+CO.sub.2 +NH.sub.2 (P)
The degradation of the rubber blades such as deterioration of surface state and reduction in the elasticity is also caused by corona products such as nitrogen oxides or ozone generated by a charger during the operation of the image forming apparatus.